Superconductors, in particular ceramic high-temperature superconductors, offer a great potential as fault current limiters which enable rapid and effective current limitation, automatic recovery, negligible impedance during normal operation and application at high voltage.
Current limiters based on high-temperature superconducting materials make use of the property of superconducting materials to switch from their superconducting state with essentially no resistance to a normal conductive and normal resistive state when at least one of its critical temperature (Tc), critical magnetic field (Hc) or critical current (Ic) is exceeded.
For example, in case of fault the current flowing through the superconductor material exceeds the critical current of the superconductor material due to large surge current and the superconducting material undergoes transition from the superconducting state to the normal conducting state. This transition is also termed “quenching”. For good operation, after the current limiting event the superconductor should have the capability to return to its superconducting state.
There are known different embodiments of superconducting current limiters. In so coiled resistive (ohmic) limiters a superconducting element becomes normally conductive and commute the current to a limiting resistance.
There are also known so called inductive current limiters. When applied with fault currents, the inductive current limiter provides a high impedance witch limits the fault currents below a threshold level. For example, U.S. Pat. No. 5,140,290 discloses a device for inductive current limiting of an alternating current, in which the current to be limited flows through an induction coil. A hollow cylinder of a high temperature superconductor is arranged in the interior of this coil, and a soft magnetic material with high permeability is arranged concentrically inside. In normal operation (rated current) the superconductivity of the hollow cylinder shields the magnetic field of the induction coil completely from the core and impedance of the induction coil is maintained at a very low level. When a fault current flows through the induction coil the superconductivity of the cylindrical body disappears and the impedance of the induction coil reaches its maximum current-limiting value.
Due to the increasing resistance during quench the superconducting material is heated up along its length. It is essential that the heating of the superconducting material along its length is as uniform as possible in order to avoid thermal destruction of the superconductor due to local overheating (“hot spots”).
For solving the problem of local overheating it was known to provide the superconductor body with a shunt of normal conducting material whose resistance is lower than the resistance of the superconducting material in its normal conducting state. In case of a sudden temperature increase current is by-passed to the shunt and heat is dissipated.
In the limiting event e. g. within 100 ms the superconducting component turns to its normal conducting state with increased resistance. At the same time current flowing through the superconducting component switches to the electrically conducting shunt. In the course thereof the shunt material is heated up.
Commonly, the electrically conducting material for the shunt is a metal such as copper or copper alloy.
In the known resistive current limiters the shunt is connected in parallel to the superconductor body.
For example, resistive current limiters are suggested which are composed of a cylindrical tubular superconductor body having a stripe of electrically conducting metal coated on its outer surface in parallel to the longitudinal axis and reaching from one end of the cylindrical tubular body to its other end. Superconductor components related to such configuration are referred to in WO 00/8657.
In DE 42 34 312 a resistive high-temperature superconductor current limiter is disclosed wherein the superconductor component has the shape of a bifilar spiral. On one face of the bifilar spiral made of the superconductor material all along its winding a metallic coating is applied for the shunt.
Since the shunts are arranged in parallel with the superconductor body both the shunt as well as the superconductor body have nearly the same length. The minimum length of the current limiter, however, is defined by the maximum electrical field along the electrically conducting shunt during short circuit. If the electrical field acting on the shunt during short circuit exceeds the maximum value overheating of the shunt occurs with temperatures exceeding the melting point of the shunt material and, consequently, the shunt melts.
For overcoming this problem the overall length of the current limiter and of the superconductor body is increased to an extent so that the electrical field acting on the shunt does not exceed said maximum value. However, this means, that, for example, in high-voltage applications high-temperature superconductor bodies are required having a very long length which in turn requires large material need with subsequent costs.
For illustration, the maximum electrical field applicable to commonly used shunt materials is about 1 V/cm. This means, that if a voltage of 10 kV is applied to the current limiter the length of the superconductor body has to be about 100 m in order to limit the electrical field to which the shunt is subjected during fault event to about 1 V/cm.
There exists a need for a resistive shunt which allows to build up sufficiently high electrical field along the superconducting body during a limiting event, it is within a very short time, usually within 100 ms, having high-voltage criteria and which can be produced in a simple and cheap way and with reduced material necessary.